Characteristics of tunable adsorbents for rate selective separation of nitrogen from methane

ABSTRACT

The present invention generally relates to a process that utilizes tunable zeolite adsorbents in order to reduce the bed size for nitrogen removal from a methane (or a larger molecule) containing stream. The adsorbents are characterized by the rate of adsorption of nitrogen and methane and the result is a bed size that is up to an order of magnitude smaller with these characteristics (in which the rate selectivity is generally 30) than the corresponding bed size for the original tunable zeolite adsorbent that has a rate selectivity of &gt;100x.

RELATED APPLICATIONS

This is a continuation-in-part application of and claims benefit ofInternational Application No. PCT/US2019/024581, filed on Mar. 28, 2019,which claimed the benefit of U.S. Provisional Application Ser. No.62/649,798, filed on Mar. 29, 2018, both of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention generally relates to adsorbent characteristicsused in a process to separate nitrogen from methane.

BACKGROUND OF THE INVENTION

Since nitrogen adsorption from methane is a relatively unexplored area,it is important to draw the background from similar adsorption processessuch as pressure swing adsorption (PSA), vacuum swing adsorption (VSA)and vacuum pressure swing (VPSA) which have been commercially utilizedfor bulk air separation, as well as trace air contaminant removal, formany decades. In PSA and VPSA processes, compressed air is pumpedthrough a fixed bed of an adsorbent exhibiting an adsorptive preferencefor one of the main constituents, typically Na in bulk air separation,CO₂ and H₂O in air prepurification, or CO and CO₂ in H₂ purification,etc., whereby an effluent product stream enriched in the lesser-adsorbedconstituent is obtained. Improvements in these processes remainimportant goals, one principal means of which is the discovery anddevelopment of better process cycles. Significant improvements have beenachieved in not only recovery of gas but also reductions in overallsystem size. These improvements also continue to provide importantbenefits even while the adsorbent being used in conjunction with thesystem is constantly improved and replaced with better alternatives.

A large majority of processes operate through the equilibrium adsorptionof the gas mixture and kinetic separations have lately attractedconsiderable attention with the development of functional microporousadsorbents and efficient modeling tools. Still, relatively few stericseparation processes have been commercialized. Kinetically basedseparation involves differences in the diffusion rates of differentcomponents of the gas mixture and allows different molecular species tobe separated regardless of similar equilibrium adsorption parameters.Kinetic separations utilize adsorbents like carbon molecular sievessince they exhibit a distribution of pore sizes which allow thedifferent gaseous species to diffuse into the adsorbent at differentrates while avoiding exclusion of any component of the mixture. Kineticseparations can be used for the separation of industrial gases, forexample, for the separation of nitrogen from air and argon from othergases. In the case of the nitrogen/oxygen separation (for example,oxygen and nitrogen differ in size by only 0.02 nm), the separation isefficient since the rate of transport of oxygen into the carbon sievepore structure is markedly higher than that of nitrogen. Hence, thekinetic separation works, even though the equilibrium loading levels ofoxygen and nitrogen are virtually identical.

Kinetically based separation processes may be operated, as noted in U.S.Patent Application Publication No. 2008/0282884, as pressure swingadsorption (PSA), temperature swing adsorption (TSA), partial pressureswing or displacement purge adsorption (PPSA) or as hybrid processescomprised of components of several of these processes. These swingadsorption processes can be conducted with rapid cycles, in which casethey are referred to as rapid cycle thermal swing adsorption (RCTSA),rapid cycle pressure swing adsorption (RCPSA), and rapid cycle partialpressure swing or displacement purge adsorption (RCPPSA) technologies,with the term “swing adsorption” taken to include all of these processesand combinations of them.

The faster the beds perform the steps required to complete a cycle, thesmaller the beds can be when used to process a given hourly feed gasflow. Several other approaches to reducing cycle time in PSA processeshave emerged which use rotary valve technologies (U.S. Pat. Nos.4,801,308; 4,816,121; 4,968,329; 5,082,473; 5,256,172; 6,051,050;6,063,161; 6,406,523; 6,629,525; 6,651,658; and 6,691,702). A parallelchannel (or parallel passage) contactor with a structured adsorbent maybe used to allow for efficient mass transfer in these rapid cyclepressure swing adsorption processes. Approaches to constructing parallelpassage contactors with structured adsorbents are known (U.S. PatentApplication Publication No. 2008/0282892). These demonstrate the benefitof having high rates of adsorption of the contaminant in equilibriumprocesses and provide the basis for why increasing rates of adsorptionhelps to intensify the process.

In the case of kinetic-controlled PSA processes, the adsorption anddesorption are more typically caused by cyclic pressure variation,whereas in the case of TSA, PPSA and hybrid processes, adsorption anddesorption may be caused by cyclic variations in temperature, partialpressure, or combinations of pressure, temperature and partial pressure,respectively. In the exemplary case of PSA, kinetic- controlledselectivity may be determined primarily by micropore mass transferresistance (e.g., diffusion within adsorbent particles or crystals)and/or by surface resistance (e.g., narrowed micropore entrances). Forsuccessful operation of the process, a relatively and usefully largeworking uptake (e.g., the amount adsorbed and desorbed during eachcycle) of the first component and a relatively small working uptake ofthe second component may preferably be achieved. Hence, the kinetic-controlled PSA process requires operation at a suitable cyclicfrequency, balancing the avoidance of excessively high cycle frequencywhere the first component cannot achieve a useful working uptake withexcessively low frequency where both components approach equilibriumadsorption values.

Some established kinetic-controlled PSA processes use carbon molecularsieve adsorbents, e.g., for air separation with oxygen comprising thefirst more- adsorbed component and nitrogen the second less adsorbedcomponent. Another example of kinetic-controlled PSA is the separationof nitrogen as the first component from methane as the second component.Those may be performed over carbon molecular sieve adsorbents or morerecently employing a hybrid kinetic/equilibrium PSA separation(principally kinetically based but requiring thermal regenerationperiodically due to partial equilibrium adsorption of methane on theadsorbent material) over titanosilicate based adsorbents such as ETS-4(U.S. Pat. Nos. 6,197,092 and 6,315,817). Thermal regeneration isdescribed as the method of passing heated gas across the adsorbent bedin order to cause desorption of the methane. In order to minimize thetime required for thermal regeneration, slow rates of methane uptake arechosen, which also correspond to the primary benefit of the ETS-4 whichis disclosed as high rate selectivity, exceeding 100× the nitrogenuptake rate over methane as the primary benefit of these adsorbents. Therelatively slow rate of uptake for nitrogen compared to an equilibriumprocess is seen as unavoidable for rate selective processes, in order tomaintain high recovery. As a result, the bed sizes to process the gasare relatively large compared to equilibrium processes.

Another patent utilizing molecular sieves for the removal of nitrogenfrom natural gas is U.S. Pat. No. 4,964,889 which discloses the use of aclinoptilolites zeolite containing magnesium cations for the removal ofnitrogen. The authors again teach the primary benefit of the zeolites ishigh rate selectivity, exceeding 100× the nitrogen uptake rate overmethane as the primary benefit of these adsorbents. Again, the slow rateof uptake of nitrogen is seen as necessary and unavoidable, in order tohave high recovery of methane and also to prevent methane poisoning.Again, as a result the bed sizes to process the gas are relatively largecompared to equilibrium processes.

SUMMARY OF THE INVENTION

The present invention generally relates to adsorbent characteristicsused in a process to separate nitrogen from methane. More specifically,the present invention relates to a process that utilizes tunable zeoliteadsorbents in order to reduce the bed size for nitrogen removal from amethane (or a larger molecule) containing stream. The adsorbents arecharacterized by the rate of adsorption of nitrogen and methane and theresult is a bed size that is up to an order of magnitude smaller withthese characteristics (in which the rate selectivity is generally 30)than the corresponding bed size for the original tunable zeoliteadsorbent that has a rate selectivity of >100×.

DETASILED DESCRIPTION OF THE FIGURES

FIG. 1. Shows the rate selectivity dependence of modified 4A on changinguptake rate of nitrogen.

FIG. 2. Outlines the TGA method sequence to measure the rates ofadsorption of nitrogen and methane.

FIG. 3. Shows an example of a TGA plot that is obtained following themethod outlined in FIG. 2.

FIG. 4. Shows an expansion of the same plot in FIG. 3 to illustrate thefeatures observed during gas switching.

FIG. 5. A breakthrough experiment diagram showing the characterizationof tunable zeolite 4A and clinoptilolite TSM-140 as compared to theminimum characteristics described herein and the ideal characteristicsfor this invention.

FIG. 6. A diagram showing a typical application of this system to anatural gas well head feed stream, post hydraulic fracturing.

DETAILED DESCRIPTION OF THE INVENTION

Separating nitrogen and methane has historically presented a challenge.While carbon-based adsorbents are readily available to adsorb methanefrom nitrogen, this leaves the methane at ambient pressure while thenitrogen is produced near the feed pressure. Typically, the methane isrequired at the feed pressure and the nitrogen at ambient pressure. Itis then preferred to adsorb the nitrogen. The present inventiongenerally relates to adsorbent characteristics used in a process toseparate nitrogen from methane.

The adsorbents of the invention are characterized by the rate ofadsorption of nitrogen and methane. These material characteristics areused in a pressure swing adsorption (PSA) process, in order to adsorbthe nitrogen and allow the methane to pass through the adsorption bed ator around the feed pressure. Demonstration of effect and benefit isshown in the examples of modeling, bench characterization and pilottesting.

In order to intensify the process and reduce the bed sizes of theadsorption system, an adsorbent having an increased rate of adsorptionof nitrogen was developed. An examination of the role of the uptake rateof nitrogen, the selectivity that was correspondingly displayed in thematerial compared to the uptake rate of methane, and the final productpurity desired, demonstrated that the long-held wisdom of higherselectivity was incorrect. What is best is a high uptake rate of thecontaminant (nitrogen in this case), and even a moderate selectivity ofaround 6x is sufficient to provide similar performance to state of theart materials that have a selectivity generally around 100×.

Previous adsorbent applications describe characteristics of adsorbentsthat are favorable or required for a separation or to improve aseparation. Previous patents have described materials that are favorablefor kinetic-controlled purification of gases or processes that arefavorable for kinetic-controlled separations. In U.S. Pat. No. 6,315,817as an example, a specification for which variant of ETS-4 or thecharacteristics required is missing, and the majority of ETS-4 productsthat can be made to fit the description do not work. Specifically,barium exchanged ETS-4 is commercially available for this separation;however the moisture content is noted to be critical to performance. Nocharacteristics exist for rate selective adsorption of nitrogen fromnatural gas (methane). The only characterization is that the benefit ofthe material is the high rate selectivity (up to and exceeding 100×) ofthe rate of uptake of nitrogen over methane. The result is very largebed sizes that have come to define the entire area of rate selectiveadsorbent processes.

This present invention defines tunable adsorbent characteristics and aprocess that allows one to reduce the bed sizes for nitrogen removalfrom a methane (or a larger molecule) containing stream. The result is abed size that is up to an order of magnitude smaller with thesecharacteristics (in which the rate selectivity is generally 30) than thecorresponding bed size for the original tunable zeolite adsorbent thathas a rate selectivity of >100×.

In one embodiment adsorbents having crystalline inorganic frameworks canbe utilized in accordance with the present invention. Crystallineinorganic adsorbents are defined as any microporous aluminosilicatehaving a regular arrangement of atoms in a space lattice. Zeolites are apreferred crystalline inorganic framework. Zeolites are porouscrystalline aluminosilicates which comprise assemblies of SiO₄ and AlO₄tetrahedra joined together through sharing of oxygen atoms. The generalstoichiometric unit cell formula for a zeolite framework is:

M _(x/m)(AlO₂)x(SiO₂)zH₂O

where M is the cation with a valence of m, z is the number of watermolecules in each unit cell, and x and y are integers such that y/x isgreater than or equal to 1. The ratio of oxygen atoms to combinedaluminum and silicon atoms is equal to 2. Therefore, each aluminum atomintroduces a negative charge of one (−1) on the zeolite framework whichis balanced by that of a cation. To activate the zeolite the watermolecules are completely or substantially removed by raising thetemperature or pulling vacuum. This results in a framework with theremaining atoms intact producing cavities connected by channels orpores. The channel size is determined by the number of atoms which formthe apertures leading to the cavities as well as cation type andposition. Changing the position and type of the cation allows one tochange and fine tune channel size and the properties of the zeolite,including its selectivity. For instance, the sodium form of Zeolite Ahas a pore size of ˜4Å and is called a 4A molecular sieve. If at least40% of the sodium ions are exchanged with a larger potassium ion, thepore size is reduced to ˜3Å. If these are exchanged with >70% calcium,one calcium ion replaces two sodium ions and the pore opening isincreased to ˜5Å. The ability to adjust pores to precisely determineuniform openings allows for molecules smaller than its pore diameter tobe adsorbed while excluding larger molecules. The Si/Al ratio can alsobe varied to modify the framework structure and provide selectivityrequired for a given separation. This is why zeolites, known asmolecular sieves, are very effective in separating on the basis of size.

Some non-limiting examples of zeolites that can be employed in thecontext of the invention include zeolite A, X, Y, chabazite, mordenite,faujasite, clinoptilolite ZSM-5, L, Beta, or combinations thereof. Theabove zeolites can be exchanged with cations including Li, Na, K, Mg,Ca, Sr, Ba, Cu, Ag, Zn, NH4+ and mixtures thereof. In one embodimentzeolite 4A is a preferred adsorbent.

The adsorbents are characterized by the rate of adsorption of nitrogenand methane. These material characteristics are used in a pressure swingadsorption (PSA) process, in order to adsorb the nitrogen and allow themethane to pass through the adsorption bed at or around the feedpressure. Demonstration of effect and benefit is shown in the examplesof modeling, bench characterization and pilot testing. Previousdisclosures on cycles do not define the rate characteristics requiredfor the cycle to work. This is an important consideration for rateselective materials as the majority of materials fail to deliversufficient separation under any process conditions even while providinga substantial difference in adsorption rates. When rate selectivitiesare disclosed for materials for this separation, they are generally 100×or higher.

Surprisingly, it has been found that lowering rate selectivity of theadsorbent allows one to reduce the bed size required to process aspecific feed stream thereby lowering cost performance. It has also beennoted that a higher rate selectivity generally corresponds to a loweruptake rate of nitrogen. When attempting to shrink the apparent poresize of an adsorbent, the decreasing rate of the larger molecule occursmuch faster than the smaller molecule. However, the decreasing rate ofuptake of nitrogen decreases the productivity of the adsorbent during afixed period of time. Thus, it is important to find a balance betweenthe two instead. One might think that increasing the rate of uptake ofthe larger molecule would cause the adsorbent to saturate with thelarger molecule. In typical rate selective separations however, steadystate adsorption is achieved relatively quickly in a matter of minutesto hours to a few days (N₂PSA, or carbon molecular sieves for example).In a scenario where it is desired to decrease the rate to the point thatthe larger molecule did not reach steady state during operation, therate of uptake of nitrogen would have to be slowed substantially suchthat the adsorbent productivity would suffer even more and would not bepractically useful. Another alternative has been to employ periodicthermal regeneration of the adsorbent, however even in this case theproductivity of the adsorbent suffers significantly. The essentiallyfinding here is that when two overlapping distributions of gas sizes arepresent, in order to find the optimal rate, one must take into accountthe rate of adsorption of the contaminant, rather than seek only higherselectivity.

In accordance with the present invention it was discovered thatcontaminant rate selective adsorbent must have a rate of contaminantuptake at least 6× greater than the product, and ideally >20× product,as characterized by the rate measurement from a gravimetric pressuremicrobalance such as a Hiden IGA unit i.e., it is not required to have arate selectivity of greater than 100× the product as taught by thestate-of-the-art in the field.

According to the invention, the adsorbent is characterized by thefollowing characteristics:

-   -   1. The rate of uptake of nitrogen is greater than 0.1 wt %/min        (0.036 mmol/g/min), in another embodiment greater than 0.4wt %        min determined by the gravimetric method going from vacuum to 1        Bar pressure of >99.9% Nitrogen (less than 0.1 ppm H₂O) at        35° C. for nitrogen measurements and from vacuum to 1 Bar        pressure of >99.9% Methane (less than 0.1 ppm H₂O) at 35° C. for        methane measurements, these targets may vary up to 50% for        differing pressures and desired product compositions, and these        numbers are based on a feed gas stream of 600 psig.    -   2. The rate of methane uptake, as characterized by a pressure        microbalance gravimetric system such the Hiden IGA, must be less        than ⅙^(th) the uptake rate of nitrogen at 1 atm >99.9% CH4, in        another embodiment less than 1/10^(th) at 1 atm >99.9% CH₄. The        rate of methane uptake should obviously not be zero and should        be greater than 1/10000^(th), in another embodiment greater than        1/1000^(th) the uptake rate of nitrogen at 1 atm >99.9% CH₄. In        one embodiment the rate of methane uptake, as characterized by        the Hiden unit, is less than ⅙^(th) but greater than        1/10000^(th) the uptake rate of nitrogen at 1 atm >99.9% CH₄, in        another embodiment less than 1/10^(th) but greater than        1/1000^(th) at 1 atm >99.9% CH₄.

In additional embodiments the adsorbent is characterized by thefollowing:

-   -   3. A heat of adsorption as determined by isotherm measurements        fit with the LRC method show that a heat of adsorption (i.e.        −A2)≥10 kcal/mol and ≤25 kcal/mol.    -   4. A heat of adsorption of methane (or a larger molecule) is        ≤200% that of nitrogen, in another embodiment from about 50% to        about 200% of that of nitrogen, and in another embodiment from        about 50% to ≤125% of that of nitrogen.    -   5. A total adsorption capacity as determined by the Hiden        gravimetric measurements that are allowed up to one week to        equilibrate, which is preferably greater than 0.4 mmol/g N₂.        Adsorption capacity of nitrogen of ≥0.2 wt %, in another        embodiment ≥0.7 wt % for a fresh sample activated at 350 to        400° C. for 8 hours under vacuum and measured in a 1 Bar        of >99.9% nitrogen at 35° C.

While these characteristics are primarily described for the separationof nitrogen from methane (natural gas), it should be noted that theywill apply to other kinetic based separations as well. The process mayalso include other adsorbents to remove a range of contaminants that arepresent in the feed stream including hydrocarbons that contain more than4 carbon atoms, moisture, carbon dioxide, sulfur containing species orother species that may reduce the working capacity of the adsorbentdescribed herein. These adsorbents could comprise activated carbon,silica, alumina, zeolites, titanosilicates, iron based, amine containingadsorbents or mixtures thereof. Typically, silica and alumina adsorbentsare used for initial water removal, followed by zeolites. Typically,titanosilicates, zeolites, activated carbon, amine containing, oriron-based adsorbents are used for sulfur removal. Typically, zeolites,titanosilicates, activated carbon, silica or amine containing adsorbentsare used for carbon dioxide removal. Typically, silica gel or activatedcarbon are used for hydrocarbon removal.

In the event that one of these adsorbents fails to remove the species,thermal regeneration may be performed to remove that species from theadsorbent described and still fall within the realm of this inventionwhich is to eliminate thermal regeneration from being used to remove theproduct gas of the invention. Methods to characterize the adsorbent aregiven below.

Pilot Description

The pilot system is a pressure swing adsorption system that operates byexploiting the difference in adsorption capacity of an adsorbent for thegas of interest over a specific pressure range. When the vesselcontaining the adsorbent is pressurized, the adsorbent will selectivelyadsorb the contaminant from the gas stream and thus remove it from theproduct stream that exits through the other end of the vessel. Whenvessel is depressurized, the contaminant will desorb and the adsorbentwill be ready to process the feed stream again. This process is madeinto a semi-continuous batch process by having 1 vessel or more than 1vessel available to process the gas at the majority of all times. Withmore than 1 vessel to process gas, additional options are available tofurther increase efficiency by retaining pressurized gas in dead volumespaces (piping or the heads of the vessels) and the process then has theability to generate a continuous stream of product.

The conceptual process flow diagram is presented in FIG. 6.

The pilot system employs multiple PSA vessels to achieve the desirednitrogen rejection and hydrocarbon recovery target. The current pilotPSA design consists of 4-6 vessels with process steps consisting of 1bed on feed and 1 bed on blowdown at a time. There are 2-3 equalizationsteps as well as product pressurization and purge steps. The pilotsystem was designed to process up to 17 kscfd and capable of using 1 to4 inch diameter beds. During the initial construction of the pilot testsystem the bed size was selected to be 1 inch due to the adsorbentperformance and with considerations of adsorbent manufacturing. Theheight was based on maximum available height in the container. Theremaining components of the design were based on similar 6 bed PSA pilotplant already in operation. Full range control valves were used for allvalves. The system was constructed entirely of stainless steel grade316. Additionally, a pretreatment system of 304 stainless steel wasdesigned and built as H2S compatible in order to remove all condensedliquids and sulfur before entering the PSA portion of the system.

LRC Description

Adsorbents were characterized using the loading ratio correlation (LRC)method as described herein and based on the article “MulticomponentAdsorption Equilibria on Molecular Sieves” by Yon and Turnock, publishedas part of the AIChE Symposium Series, 117, Vol. 67, in 1971 inAdsorption Technology. Isotherm measurements were performed by using anIGA balance as described below, for temperatures of 20° C., 35° C. and50° C.

Hiden IGA Description (Equilibrium and Rate)

Rate and equilibrium characterization of samples were performed using aHiden IGA pressure microbalance (Model#HAS022650) which measures singlecomponent gas uptake and was used to examine the adsorption of N₂ andCH₄. The samples were loaded and gas adsorptions were measured asinstructed in the IGA Systems User Manual #HA-085-060. Each sample wasloaded and activated in situ under vacuum with a temperature ramp of 0.7C/min to between 350 and 400° C. and held for 12 hours. It was thencooled to the adsorption test temperature at a rate of 1° C./min. Theamount of gas adsorbed by the adsorbent is measured in micrograms at afixed temperature controlled by a constant temperature bath. Thepressures points are taken from 0.1 bar to 10 Bar allowing up to 7 daysto reach equilibrium. Equilibrium and leak check verification is done bya desorption isotherm that matches the adsorption isotherm. A buoyancycorrection was determined using helium and this was used to adjust themicrogram weight for buoyancy effects using the molecular weight of thegas being measured. The buoyancy corrected microgram weight was used tocalculate uptake using standard methods and using the activated sampleweight. For rate measurements, the test gas (N₂ or CH₄) was introducedat 1 Bar then the sample was held at pressure recording the weight as afunction of time. System dynamics require approximately 2 minutes tostabilize. Weight data after 2 minutes were corrected for buoyancy andconverted to uptakes in weight % or mmol/g and the uptake versus timedata were fit to a first order process to obtain rates. Each materialwas tested first for N₂, prior to being reactivated before repeating thetest using CH₄.

Breakthrough Description

A breakthrough test system was created to test the adsorbent samplesusing a 12″ long 1″ pipe filled with adsorbent. A breakthrough test wasrun by first saturating the bed with a flow of 300 sccm at 400 psig of99% methane (where methane is >99.99%) and 1% helium (where heliumis >99.99%) gas for 2 hours, then a flow of 300 sccm of a49.75/49.75/0.5 mixture of N₂ (where nitrogen is >99.99%)/CH₄/He wasintroduced as a feed gas to the adsorbent bed and the outlet gas wasmeasured using a gas chromatography mass spectrometer. The breakthroughwas recorded as a nitrogen breakthrough example. After 30 minutes thisflow was switched to 300 sccm of 99% nitrogen and 1% helium and held for2 hours. Then the flow was switched back to the 300 sccm of49.75/49.75/0.5 mixture of N₂/CH₄/He and this was recorded as themethane breakthrough. These breakthrough curves were then used withgPROMS software provided by Process Systems Enterprise, Inc. (PSE) toautomatically perform parameter estimation of a model that was createdas a replica of the system. The libraries supplied with the adsorptionaspect of Process Builder from PSE are sufficient to replicate theseresults. A detailed description and instructions on how to perform thesesimulations is provided by PSE.

Modeling Description

The results from the breakthrough test and parameters obtained from themodeling were used with the methodology described by Mehrotra, et al. inArithmetic Approach for Complex PSA Cycle Scheduling, Adsorption, 2010,pp. 113-126, vol. 16, Springer Science+Business Media which details thebasis for modeling PSA processes. These simulations were performed usingProcess Builder, from PSE.

TGA Rate Measurements

A TGA method was developed to assess comparative nitrogen rates thatinvolves both an in-situ activation step followed by adsorption testsusing oxygen and nitrogen at 25° C. The thermogravimetric method using aTA Instruments Q500 system installed in a glove box to minimize theimpact of air leaks. Nitrogen, and oxygen, gases supplied to theinstrument were high purity. The balance purge gas and gas 1 wasnitrogen and a gas 2 corresponds to oxygen. For all experiments, abalance purge of 5 cc/minute was used and the gas directly over thesample was set to 95 cc/minute (nitrogen or oxygen). A samplingfrequency of 0.5 sec/point was used for all adsorption steps. Aluminapans were used for all studies and the sample size after activation wasin the range 100 to 120 mg. The sample activation was performed byheating the sample under nitrogen purge at 2° C. per minute to 150° C.,maintaining isothermal for 60 minutes, heating at 5° C./minute to 350°C., holding at 350° C. for 120 minutes, then cooling to 25° C. Thenitrogen equilibrium capacity at atmospheric pressure and 25° C. isreported as the weight gain on cooling under nitrogen relative to theminimum weight at 350° C. (the activated sample weight). An assessmentof relative rate for different samples and preparation is captured byswitching from nitrogen to oxygen. A transient weight gain is observedfollowed by a drop attributable to oxygen uptake followed by nitrogenleaving. A corresponding switch from oxygen back to nitrogen results ina transient weight loss followed by a weight gain attributable to oxygenloss followed by nitrogen pickup. Values reported as “nitrogen uptakerate” correspond to the maximum slope observed in the nitrogen uptakeportion and is equivalent also to the peak in the derivative weight withrespect to time for the same step. Values are reported in weight%/minute. Rate measurements for selectivity determinations reliedexclusively on a Hiden pressure microbalance rather than the TGA method.

EXAMPLE 1. MODELING RESULTS FOR HIGHER SELECTIVITY RATIOS

This example demonstrates that once the rate selectivity is above 30,the ratio of uptake rates (N₂/CH₄) as measured via Hiden microbalance,does not significantly impact performance until a ratio of uptake ratesis greater than 1,000,000 which has not been achieved in an economicallyviable offering. At a ratio of 1, the system works against the desiredseparation to instead produce a purified product of nitrogen. At ratiosabove 5, the adsorption of methane becomes too low on a normal cycle andthe product of purified methane begins to emerge. At ratios above 35 theadsorption of methane fails to negatively impact the performance of thesystem with proper process cycles and the system performs at peakperformance for the majority of selectivity ratios studied. Theexception is that above a selectivity of 1,000,000, then the adsorbentdoes not reasonably saturate with methane during the expected lifetimeof the system (>5 years) and thereby increases the working capacity ofnitrogen almost 100% vs CSS conditions. Selectivity ratios of almost1,000,000 have never been reported in literature and are currently˜10,000 times higher than the state of the art.

Ratio of rates of uptake BSF at year 5 (N₂/CH₄) (lbs/MMscfd Feed) 1 — 103600 35 1200 100 1200 1000000 1200 10000000 600 For 35% N₂ in feed to20% N₂ in product at a recovery of 80% at 35 C.

EXAMPLE 2. MODELING RESULTS DEMONSTRATIONG REDUCED SELECTIVITY, ANDHIGHER UPTAKE OF NITROGEN BENEFIT

The commercial performance of the tunable zeolite 4A was modeled with arelative rate of 0.9 wt % N₂/min characterized material. The recovery isthe total hydrocarbons recovered from the 4-bed system. The productionis the relative production of the system at different conditions. Thepurity is the methane concentration of the product. The N2 rate is therate of uptake of nitrogen on the material relative to the 0.9 wt %N₂/min uptake rate material. The CH₄ rate is the rate of uptake ofmethane on the modeled material relative to the same material basiswhich was 0.03wt % CH₄/min. The feed concentration is 35% N₂, 65%Methane at 35° C.

TABLE 1 the rate selectivity dependence of modified 4A (WO201715431164)for changing uptake rate of nitrogen. Recovery Purity Production N₂ RateCH₄ Rate 66% 90% 100%  100% 100% 60% 90% 90% 100% 200% 47% 90% 71% 100%400% 52% 90% 99%  20% 100% 21% 90% 95%  4% 100% 74% 90% 160%  200% 200%— 55% — 800% 800% 53% 90% 90%  20%  20%Table 1 show that doubling the uptake rate of methane decreases therecovery of a fixed bed size system, but only by ˜10%. This demonstratesthat higher selectivity only has a marginal benefit in this regime.

In the case where the rate of uptake of both nitrogen and methane aredoubled the theoretical performance is significantly higher, howeverthis assumes the rate selectivity ratio is maintained, whichunfortunately it is not. This demonstrates that higher uptake rates ofnitrogen are preferred, as described herein.

A 400% increase in methane uptake begins to lower the recoverysubstantially more, however the material is still viable for theseparation. This further demonstrates that higher selectivity is not themost important consideration even at the edges of the proposedcharacteristics described herein.

If an 800% increase to both rates is modeled, the material is no longerable to maintain a product purity, and instead begins to remove methanefrom nitrogen. This is an effect of accounting for physical restrictionswithin the system related to valve open speeds and gas flows across theadsorbent. If a system were designed to mitigate these, the higher ratesof adsorption could be tolerated, and the system size could be reducedeven more. However, it is important to note here that typical adsorptionsystems have physical limits that render them unable to utilize suchhigh rate materials and the practical design point for materials whenconsidering these factors is a slower uptake rate of nitrogen andmethane.

In the case of lowered rates of adsorption of nitrogen but maintained orlowered rates of adsorption of methane we see a drop in the processperformance, but the performance is not affected by the rate of methaneuptake in this regime. This suggests that at a certain point, slowingthe uptake rate of methane no longer benefits the process as shown inexample 1. At a certain point, the rate of uptake of nitrogen issufficiently slow that the process begins to perform very poorly at afixed bed size and can only be remedied with very costly increases tothe bed size. This demonstrates the improvement discovered here ofincreased rates of nitrogen uptake that the expense of even largerincreases to the rate of methane uptake.

EXAMPLE 3. FASTER SYSTEM RESPONSE

Since one of the benefits of higher selectivity is the increased time toreach cyclic steady state (CSS), it's important to note that CSS isreached significantly faster with these new material characteristics. Adefining characteristic of state-of-the-art materials is strongcompeting adsorption via methane which results in a lowered workingcapacity/minimal working capacity after adsorbent saturation. In anormal PSA cycle with ETS-4, very large adsorbent beds must be utilizedwith very low recovery systems to generate a moderate purity product. Tocounter this, one can implement methods to desaturate the adsorbent andbalance the economics of large beds with low recovery (normal) or highcapital (with desaturation). One example of desaturation is thermalregeneration. U.S. Pat. No. 6,444,012 to Dolan et al describes a methodto desaturate the adsorbent by heating the product stream (largelymethane) in order to force methane out of the pores via a TSA process.This consumes methane, energy (for heating) and requires replacementcapital of the beds not undergoing thermal regeneration in order tomaintain continuous operation. Additionally, this requires that theadsorption rate of methane is very slow. A very slow adsorption rate ofmethane usually is associated with a slow adsorption rate of nitrogen.CSS loading capacity of methane on Tunable 4A was reached inapproximately 30 minutes via modeling and pilot experiments for thehighest performing adsorption rates tested. This has additional benefitssuch as being able to respond rapidly to changing feed conditions. Oiland gas wells typically have significant fluctuations and variations.Responding to these is an additional benefit of faster adsorptionrate-based processes.

wt %/min N2 CSS Time Pilot CSS Time model uptake (min) (min) 1.2 20 22.50.9 30 30 0.6 120 45 0.1 — 270

EXAMPLE 4. LOW RATE SELECTIVITY, HIGH UPTAKE RATE ADSORBENTS

A material was made to demonstrate the proposed benefit of higher uptakerates of nitrogen even at reduced overall rate selectivity,demonstrating the benefit illustrated by the model in example 1.

23.00 lbs. of zeolite 4A powder supplied by Jianlong (as 4A-D) on a dryweight basis (29.50 lbs. wet weight) was placed in a WAM MLH50 plowmixer. With the mixer agitating, 2.16 lbs of MR-2404 (a solventlesssilicone containing silicone resin from Dow Corning) was pumped in atrate of 0.07 lb/min. After the MR-2404 addition was completed, 9.2 lbsof water was added at a rate of 0.3 lb/min under constant stirring inthe plow mixer. At the end of the water addition, plow mixing wascontinued for an additional 5 minutes. The plow mixed powder productlabeled hereinafter “the formulation” was transferred to a tiltedrotating drum mixer having internal working volume of ˜75 L and agitatedtherein at a speed of 24 rpm. Mixing of the formulation was continuedwhile beads were gradually formed which had a porosity, as measuredusing a Micromeritics Autopore IV Hg porosimeter on the calcinedproduct, in the 30-35% range. The beads were subjected to a screeningoperation to determine the yield and harvest those particles in the 8×16U.S. mesh size range. The product beads were air dried overnight priorto calcination using a shallow tray method at temperatures up to 595° C.The shallow tray calcination method used a General Signal Company Blue-Melectric oven equipped with a dry air purge. ˜500 g. dry wt. of the 8×16U.S. mesh adsorbent was spread out in a stainless steel mesh tray toprovide a thin layer. A purge of 200 SCFH of dry air was fed to the ovenduring calcination. The temperature was set to 90° C., followed by a 6hour dwell time. The temperature was then increased to 200° C. graduallyover the course of a 6 hour period, and further increased to 300° C.over a 2 hour period and finally increased to 595° C. over a 3 hourperiod and held there for 1 hour before cooling to 450° C. after whichthe adsorbent was removed, immediately bottled in a sealed bottle andplaced in a dry nitrogen purged drybox. The calcined beads wererescreened to harvest those particles in the 8×16 U.S. mesh range.

Characterization of the tunable 4A samples calcined at 595° C. wasperformed using a thermogravimetric screening method as describedearlier in “TGA description”. The nitrogen uptake rate as performed inthe test was determined to be ˜0.2 weight %/minute as measured using theTGA method disclosed herein. When the product beads in Example 1 werecalcined up to 575° C., the nitrogen uptake rate as performed in thetest was determined to be ˜0.7 weight %/minute as measured using the TGAmethod disclosed herein. Subsequently, when the product beads in Example1 were calcined up to 555° C., the nitrogen uptake rate as performed inthe test was determined to be ˜1.2 weight %/minute as measured using theTGA method disclosed herein.

Breakthrough Data from Model and Lab Experiment

The breakthrough data demonstrates the achievement of the required ratecharacteristics, and is shown in the FIG. 5 by the minimum requiredrates of adsorption for nitrogen, and maximum rates of adsorption ofmethane, and the ideal rates of adsorption for both of these. It isclear in the FIG. 5 that the actual adsorbent had rates of adsorption ofboth components in between these two extremes. These two extremes alsodetermine the characteristics alternative materials need to meet inorder to have high performance in this process. Also shown in the FIG. 5is clinopotilolite (clino) TSM-140 which is commercialy available. Thisstate of the art material does not have the uptake rate of nitrogen tomeet the characteristics described here.

Pilot Data

The relative rate of uptake correlates to the TGA measurement. Theselectivity is the rate of uptake of nitrogen divided by the rate ofuptake of methane as determined by the breakthrough test and modelfitting. The pilot recovery is the recovery observed in the pilot systemfor a feed concentration of 35% nitrogen, a flow rate of 120 scfh usingfour 1″ beds filled 5.5′ tall. The product impurity was held at 20%nitrogen. The results show that with increasing selectivity, the pilotrecovery falls significantly, due to the inability of the material toprocess enough gas to overcome the losses from void spaces. This is anunfortunate reality of high selectivity is that is typically correspondsto reduced uptake rate of nitrogen. When this is considered, therecovery rises substantially, up to a point that the process is unableto take advantage of the higher rate of uptake to due to low responsetime of the system and the fast uptake of methane which ultimately bothwork to lower recovery. One way that others have overcome this lowrecovery is to increase the bed size, thereby increasing the amount ofgas the material is able to process and offsetting the void spacelosses. Since this modification is no longer necessary, this recoverygain is equivalent on a commercial scale to lower bed sizes.

TABLE 2 showing the impact of relative uptake rate on selectivity and onfinal recovery in the pilot system. N₂ rate of uptake Rate SelectivityPilot wt %/min (N₂/CH₄) Recovery 0.1 120  2% 0.6 70  6% 0.9 44 24% 1.230 20%

EXAMPLE 5. MODELING SENSITIVITY TO PRODUCT PURITY AND PRESSURE

Another study was conducted to determine the optimal adsorbent for aproduct purity of 5% nitrogen compared to 20% nitrogen and for a feedstream of 200 psig compared to 600 psig. The results show that somevariation in the preferred adsorption uptake rate of nitrogen and thesubsequent ratio of uptakes compared to methane exists, but that thisvariation is typically limited to +/−50% between applications.Additionally, while the optimal target can vary up to 50%, theperformance for a 50% variation in uptake rates does not generally causea larger variation in process performance.

TABLE 3 Model N₂ rate of Rate Projected uptake Selectivity Pilot FeedProduct wt %/min (N2/CH4) Recovery impurity impurity 0.1 120  4% 35% 20%0.6 70  8% 35% 20% 0.9 44 28% 35% 20% 1.2 30 22% 35% 20% 0.1 120  8% 10% 5% 0.6 70 34% 10%  5% 0.9 44 32% 10%  5% 1.2 30 28% 10%  5%Table 3 shows that for different processing feed impurities and desiredproduct impurity levels, the optimal adsorption rate of nitrogen canvary accordingly. In particular, as the impurity level is reduced, theimpact of selectivity becomes more important and the importance of theuptake rate of nitrogen begins to fall. This should be considered whenselecting the optimal characteristics for the process. It also leads tothe prospective of multiple layers of varying uptake rate tunablezeolite 4A adsorbents in order to best accomplish a separation processwhen there is a potential large variation in impurity levels through theprocess, as a method to best reduce the overall bed size of the system.

We claim:
 1. A pressure swing adsorption process for kinetic separationof N₂ from a feed stream comprising at least methane and N₂, saidprocess comprising feeding the feed stream to an adsorbent bedcomprising adsorbent having: a rate of adsorption of at least 0.036mmol/g/min for N₂ as determined by the Hiden method and a rate ofadsorption of methane that is ⅙^(th) to 1/10000^(th) the adsorbent'sadsorption rate for N₂ as determined by the Hiden method and recoveringa product stream containing said at least methane gas with a reducedlevel of N₂.
 2. The process of claim 1 wherein the adsorbent has anadsorption rate of at least 0.143 mmol/g/min for N₂ impurity asdetermined by the Hiden method and an adsorption rate for methane thatis 1/10^(th) to 1/1000^(th) of the adsorption rate for N₂ as determinedby the Hiden method.
 3. The process of claim 1 wherein said adsorbentcomprises zeolite A, X, Y, chabazite, mordenite, faujasite,clinoptilolite, ZSM-5, L, Beta, or combinations thereof.
 4. The processof claim 3 wherein said adsorbent is a zeolite exchanged with at leastone cation selected from Li, Na, K, Mg, Ca, Sr, Ba, Cu, Ag, Zn, NH4+ andcombinations or mixtures thereof
 5. The process of claim 1 wherein saidadsorbent is zeolite A.
 6. The process according to claim 2 where thefeed stream that may contain additional gas species such as ethane,propane, butane and hydrocarbons with more than 4 carbon atoms and mayinclude adsorbents to remove said hydrocarbons.
 7. A process accordingto claim 2 where the feed stream that may contain additional gas speciessuch as water, carbon dioxide or sulfur species and may includeadsorbents to remove said species.
 8. An adsorbent for the kineticseparation of N2 impurity from a feed stream comprising at least methaneand nitrogen gas, said process comprising feeding the feed stream to anadsorbent bed comprising an adsorbent having: a rate of adsorption of atleast 0.036 mmol/g/min for N₂ as determined by the Hiden method, and arate of adsorption for methane that is ⅙^(th) or less than theadsorbent's adsorption rate for N₂ as determined by the Hiden method. 9.The adsorbent of claim 8 wherein the adsorbent has an adsorption rate ofat least 0.143 mmol/g/min for said N₂ as determined by the TGA methodand an adsorption rate for the methane that is 1/10^(th) or less of theadsorbent's adsorption rate for N₂ as determined by the Hiden method.10. The adsorbent of claim 8 which comprises zeolite A, X, Y, chabazite,mordenite, faujasite, clinoptilolite, ZSM-5, L, Beta, or combinationsthereof
 11. The adsorbent of claim 10 wherein said adsorbent is azeolite is exchanged with at least one cation selected from Li, Na, K,Mg, Ca, Sr, Ba, Cu, Ag, Zn, NH4+ and combinations or mixtures thereof.12. The adsorbent of claim 10 wherein said adsorbent is zeolite A.